Take a look at WINCC/Connectivity pack manual. they have few examples there.

here is query that will probably work for you:

The following query uses the <TimeStep> parameter and returns all values of ValueID "1" and
"2" - starting from start time "TimeBegin" till 2 minutes later in intervals of "15" seconds with
the aggregation type "261" = "Average value with linear Interpolation".
"TAG:R,(1;2),'2004-10-13 17:00:00.000','0000-00-00 00:02:00.000',
'TIMESTEP=15.261'"

It sound little strange to me that your customer wants to see data in different time stamps. from my experience they more interested in logging average or max/min values at different intervals. Anyway you can get this info with the above query by simply specifying aggregation type